The invention relates to rotating polygon mirrors used for scanning an optical beam in bar code readers.
FIG. 6 is a perspective view showing a construction of an optical beam scanner using a conventional rotating polygon mirror. In, FIG. 6, a rotating polygon mirror 1 is formed of a regular polygonal column, and has, on the respective side surfaces thereof, a plurality of reflective surfaces 11 (11a, 11b, 11c, 11d, . . . ) arranged so as to be rotationally symmetrical with one another relative to the axis 2 of the regular polygonal column.
A light source 3 includes a laser diode 3a and a convergent lens 3b. An optical beam 4 from the light source 3 is projected obliquely onto one of the reflective surfaces 11 of the rotating polygon mirror 1, e.g., the reflective surface 11a, and is reflected by the reflective surface 11a to irradiate a subject area 5. Since the rotating polygon mirror 1 is rotated in a direction of an arrow P around the axis of the regular polygonal column (hereinafter referred to as the "rotating axis") 2 by a drive unit (not shown), a reflecting surface sequentially change in the order of the reflective surfaces 11a, 11b, 11c, 11d, . . . , causing the optical beam 4 to move in a direction of an arrow Q within the subject area 5 every time the reflective surface changes.
Optical beam scanners of this type are used, e.g., for bar code readers. In this case, the subject area 5 is a bar code directly printed on a label bonded onto a product or the surface of a product. The subject area 5 is scanned by the optical beam, and the reflective light is received by a not shown light receiving device to thereby read the bar code data by photoelectric conversion.
The above-mentioned rotating polygon mirror is formed by cutting a solid body (e.g. aluminum body) into a regular polygonal column and polishing the respective side surfaces of the regular polygonal column so that such side surfaces are formed into reflective surfaces. The polishing process is performed by machining. (R.multidot.D KOBE STEEL ENGINEERING REPORTS/Vol.39 No.4, PP39-42 (1989), "Precision Machining of High-Quality Polygonal Mirrors") Thus, this process is not suited for mass production, and is disadvantageous in terms of cost. In the meantime, the process of fabricating a regular polygonal column by resin molding and forming the reflective surfaces by metal deposition or plating the side surfaces of the regular polygonal column has been developed. (JAPAN STEEL WORKS ENGINEERING REPORT No.46, PP 94-98, "Injection Molding of a Polygon Mirror") These methods are suitable for mass production, but address the problem that peel-off, discoloration, cracks, or the like are easy to occur due to the metal deposition or the plating being affected by the ambient temperature and humidity.
Further, the function of the rotating polygon mirror is limited to constant-angle scanning, giving functions such as convergence and divergence of an optical beam or constant-velocity linear scanning of the optical beam required to be performed by laser beam printers, or the like to an optical lens that is independent of the rotating polygon mirror. Therefore, downsizing and cost reduction of devices thus applied have been difficult.